1. Field of the Invention
The invention relates to an optically controlled flip-flop, especially a toggle flip-flop. In many time-discrete systems, a phase noise of the clock signal limits the power. With analog-digital converters, a phase noise of the clock signal leads to a noise in the sampled signal. In the case of high frequencies, this noise predominates over noise sources elsewhere in the signal path. The phase noise of the clock signal in communications systems limits the lower limit of the time interval between two successive symbols. Phase noise in a clock signal originates partly within the oscillator which generates the clock signal and partly through noise in the further circuit which distributes the clock signal.
2. Discussion of the Background
A clock signal is conventionally generated through an electronic oscillator, for example, a quartz oscillator. However, with such an oscillator, the intrinsic phase noise of the oscillator, which leads to a phase noise of the generated clock signal is disadvantageous.
Moreover, the document E. W. Jacobs, “Optically Clocked Track-and-Hold for High-Speed High-Resolution Analog-to-Digital Conversion”, Proc. IEEE International Topical Meeting on Microwave Photonics, 2004, pages 190-192, shows a holding element (sample-and-hold device) comprising a diode bridge. In this context, sampling pulses are generated by means of a mode-locked laser (mode-locked laser), the output of which is supplied simultaneously to two photodiodes. In the absence of light, two current sources conduct a zero-signal current to the diode bridge. Meanwhile, the output of the diode bridge follows an analog input signal to be sampled. When a laser pulse strikes the photodiodes, the two photodiodes interrupt the zero-signal current, which is generated by the current sources, and accordingly deactivate the diode bridge. The output of the diode bridge is therefore held constant during the time duration of the laser pulse. To allow digitization by an analog-digital converter, in view of the shortness of the laser pulse, the output signal of the diode bridge must be additionally sampled by a secondary holding element in order to increase the time duration. The requirements for the phase noise of this secondary holding element are significantly reduced by comparison with the requirements for a primary holding element. An electronic generation of the clock of the secondary holding element is therefore possible. With the holding element shown in that context, it is disadvantageous that it requires two photodiodes.